Ethernet was designed for office environments, where distances between users (and distances between users and networking hubs) are relatively short (˜100 meters). Ethernet relies on a shared-bus architecture which is not particularly suited to applications where distances between users are substantially longer, such as in residential applications. In residential applications, a service provider such as a telephone company or an MSO (CATV service provider) might want to have its active equipment located in a central office or hub, which could be up to 20 km from subscribers.
When considering Fiber-To-The-Home (FTTH) or Fiber-to-the-Curb (FTTC) systems, one approach has been a type of network called a Passive Optical Network (PON). A general review of PONs can be found in an article by David W. Faulkner, et al. “Optical Networks for Local Loop Applications,” J. Lightwave Technol. 7, pp. 1741–1751 (1989) and in an article by Yih-Kang Maurice Lin and Dan R. Spears, “Passive Optical Subscriber Loops with Multiaccess,” J. Lightwave Technol. 7, pp. 1769–1777 (1989).
A schematic view of a typical PON is shown in FIG. 1. The PON connects a head-end 10 with a multiplicity of users 30 using passive optical components, such as optical fibers 50, 60 and splitters 40. This avoids having to provide power or maintenance to active components located in the field (often referred to as the “outside plant”, or OSP). Generally, one or two feeder optical fibers 50 are run from the head-end 10 out to a passive splitter 40, which distributes the light from the head-end to drop fibers 60 that run to individual users. (For simplicity, FIG. 1 shows one drop fiber 60). At the end of each drop fiber 60, an active component called an Optical Network Unit (ONU) 20 converts the optical signal to an electrical signal for delivery to the customer. At the customer site, various types of Customer Premise Equipment (CPE) 30, such as telephones, computer modems, or televisions, can be attached.
PONs are cost-effective largely because the expensive distribution fiber and head-end equipment is shared over a large number of users. If only a single distribution fiber 50 is used for upstream (ONU to head-end) and downstream (head-end to ONU) transmissions, some means for separating the upstream and downstream signals is required. Similarly, some means is also required for separating the signals of the multiplicity of users. For example, in accordance with the Time Compression Multiplexing (TCM) protocol developed by NTT, time is divided into fairly long periods (˜1 ms) called “frames”. In the first half of each frame, the head-end sends information out to the ONUs and in the other half of the frame, the ONUs send information to the head-end. The downstream portion of the frame is further subdivided into time slots in which the head-end sends packets addressed to individual ONUs. In the upstream portion of the frame, each ONU is given a time slot in which it is allowed to transmit.
Another approach is Subcarrier Multiple Access (SCMA). SCMA techniques in PONs are described in T. H. Wood, et al., “Demonstration of a Cost-Effective Broadband PON System,” Phot. Technol. Lett. 6, pp. 575–578 (1994).
PONs can be configured to carry telephony, video, and data. See, for example, T. H. Wood et al., “Cost-Effective FTTH System Providing Broadband Data over Cable Modems along with Analog and Digital Video,” Proceedings of the Optical Fiber Communication Conference, 1998, paper PD-28; and T. H. Wood et al., “FiberVista: A Cost-Effective Fiber-to-the-Home (FTTH) System Providing Broad-Band Data Over Cable Modems Along with Analog and Digital Video,” IEEE Phot. Ltrs., Vol. 11, No. 4, April 1999. In the systems described therein, analog and digital video are transported downstream on a PON using a format appropriate for direct connection into a cable-ready TV or a digital set-top-box. Data is transmitted in a format ready for connection to a cable modem.
Both Ethernet and PONs are designed to operate on a shared medium—that is, many users share the same physical medium, and thus must avoid interfering with each other. Typical PONs and Ethernet solve this problem in completely different ways. Because the known protocols for carrying data on PONs (e.g., TCM or cable-modem signaling) are designed specifically for the PONs and are not related to native Ethernet signaling, conventional PONs require complicated interfaces between the PON and an Ethernet Network Interface Card (NIC).
As described above, a common approach for upstream transmission in a PON is to have the head-end allocate to individual ONUs time slots during which each ONU can talk. This is termed Time-Division Multiple Access (TDMA). If each ONU talks only when allowed, no collisions will occur. Ethernet, however, which is designed for shorter distances uses another approach: Carrier Sense Multiple Access/Collision Detection (CSMA/CD). The operation of Ethernet is set forth in the IEEE 802.3 specification (which is described in R. Breyer et al., “Switched and Fast Ethernet”, 2nd Ed., Ziff-Davis Press, Emeryville, Calif.) In accordance with CSMA/CD, each NIC listens to the bus before talking. If the bus is not in use, a NIC speaks (i.e., transmits a packet). Usually, the transmitted packet will get through to its destination. Nonetheless, there is still the possibility of a collision. This could occur when a first NIC (NIC1) talks, yet the bus appears available to a second NIC (NIC2) for a period of time after NIC1 has begun to talk. This period of time is due to the finite amount of time required for the packet transmitted from NIC1 to reach NIC2.
To solve this collision problem, the NICs continue to listen to the bus while they are talking. If a NIC senses someone else talking, the NIC stops talking and assumes that its packet did not make it through. The NIC then waits a short time and retransmits.
The CSMA/CD scheme works well if the time it takes for information to propagate from one NIC to another is short compared to the time it takes to transmit the shortest packet. If this condition is violated, NIC2 might finish transmitting its packet before the packet from NIC1 arrives, and NIC2 would think its packet got through when in fact it did not. Thus, Ethernet operation becomes problematic as the physical size of the bus (and thus the time it takes for a packet to propagate from one NIC to another) increases. This is one reason why Ethernet links are typically defined for distances of approximately only 100 m and why extending those distances to ranges on the order of 20 km has thus far not been practicable.